CH. V] WASPS 129 



hooks have become engaged in the ledge of the fore- wings, 

 in which case they partake in the movements of the 

 latter. It is therefore only necessary to consider the 

 motor mechanism of the fore-wings. There are no muscles 

 attached directly to the wing itself, but its vibratory 

 movements are brought about by alterations in the shape 

 of the framework of the meso-thorax. These changes are 

 effected by two sets of powerful muscles ; a longitudinal 

 set from the anterior to the posterior end of the meso- 

 thorax occupying the dorsal two-thirds of the median 

 region, and a paired transverse set, to right and left of 

 the first, running nearly vertically from the dorsal to the 

 ventral surface. The articulation of the extended wing 

 may, for the purpose of understanding the principle of the 

 mechanism, be regarded as a plate at right angles to 

 the wing and situated about half-way along the side of the 

 meso-thorax at the ventral end of the hinge between the 

 dorsal parts of the anterior and posterior portions of the 

 meso-thorax (scutum and scutellum). This "wing-plate" 

 must be considered as firmly attached to both scutum and 

 scutellum. When the longitudinal muscles contract they 

 draw the anterior part of the scutum and the posterior 

 part of the scutellum towards each other. This shortening 

 of the meso-thorax results, in consequence of the above- 

 mentioned hinge and of the thrust against certain parts 

 of the internal skeleton, in the sides of the hinge getting 

 pinched together and arching in a dorsal direction; i.e. the 

 meso-thorax becomes deeper dorso-ventrally. The hinge 

 in its dorsal movement draws with it the "wing-plate," 

 throwing the latter through about a right angle from an 



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